I think Simon's on the right track... sounds like a potential caching issue. Are you using a caching proxy server? Can you replicate the problem when you connect directly to the web server?
Check out this thread for more information on this:
https://openacs.org/forums/message-view?message_id=27295
We had a similar issue in the past and it was solved by placing the following two lines in our master template:
ns_set put [ns_conn outputheaders] "Cache-Control" "private"
ns_set put [ns_conn outputheaders] "Expires" "Thu, 01 Jan 1998 07:00:00 GMT" (or some date in the past)
Hope this helps...
